1. Field of the Invention
The present invention relates to liquid crystal display units and, more specifically to a liquid crystal display unit capable of enhancing visibility at a large angle of view.
2. Description of the Background Art
A Liquid crystal display unit has conventionally been known as one type of display unit. One of major technological subjects of the liquid crystal display unit is to increase its angle of view. A liquid crystal display unit which has an IPS mode (In-Plane Switching mode) as one of modes for obtaining such large angle of view has been disclosed for example in Japanese Patent Laying-Open No. 8-254712. The liquid crystal display unit of an active matrix type having the IPS mode makes a direction of an electric field applied to a liquid crystal parallel to a substrate.
FIG. 18 is a schematic cross-sectional view showing a conventional liquid crystal display unit having an IPS mode, where a voltage is not applied to its electrode. Further, FIG. 19 is a schematic plan view showing the liquid crystal display unit having the IPS mode shown in FIG. 18. Referring to FIGS. 18 and 19, the liquid crystal display unit having the IPS mode will be described.
Referring to FIG. 18, the liquid crystal display unit having the IPS mode is provided with a back light member 101 and a liquid crystal display 103. Liquid crystal display 103 is formed above back light member 101. Liquid crystal display 103 includes polarizer 104a and 104b, glass substrates 113a and 113b, electrodes 106a and 106b, a source line 109, orientation films 107a and 107b and liquid crystal molecules 108a and 108b. Glass substrate 113a is formed on polarizer 104a. Linear electrodes 106a, 106b and source line 109 are respectively formed on and above glass substrate 113a. Orientation film 107a for adjusting an orientation direction of liquid crystal molecules 108a and 108b (a major axis direction of liquid crystal molecules 108a and 108b) is formed above electrodes 106a and 106b. Glass substrate 113b is formed above orientation film 107a with a liquid crystal including liquid crystal molecules 108a and 108b interposed. The other orientation film 107b is transferred on glass substrate 113b. Polarizer 104b is formed on glass substrate 113b. 
Here, as shown in FIG. 19, when the voltage is not applied to electrodes 106a and 106b, the orientation direction of liquid crystal molecules 108a and 108b is adjusted to have some angle with respect to a direction in which electrodes 106a and 106b extend. It is noted that anisotropy of dielectric constant of the liquid crystal is assumed positive.
Referring further to FIG. 18, polarizer 104a and 104b are arranged with their transmission axes orthogonal to each other. It is noted that the transmission axes mentioned here correspond to directions of vibration components of light allowed to be transmitted through polarizer 104a and 104b. Then, the transmission axis of polarizer 104a positioned between back light member 101 and liquid crystal molecules 108a, 108b is aligned with the orientation direction of liquid crystal molecules 108a and 108b when the voltage is not applied to electrodes 106a and 106b. When the voltage is not applied to electrodes 106a and 106b, birefringence does not occur at liquid crystal molecules 108a and 108b. So the light directed from back light member 101 through polarizer 104a dose not transmit the polarizer 104b. As a result, a black screen (hereinafter represents a state in which a screen appears black) is achieved.
FIG. 20 is a schematic cross sectional view showing the liquid crystal display unit having the IPS mode shown in FIG. 18, where the voltage is applied to the electrode. Further, FIG. 21 is a schematic plan view showing the liquid crystal display unit shown in FIG. 20. Referring to FIGS. 20 and 21, an operation of the liquid crystal display unit having the IPS mode when the voltage is applied to the electrode is described.
Referring to FIGS. 20 and 21, when the voltage is applied to electrodes 106a and 106b, an electric field is formed in the direction indicated by an arrow 110. When such electric field is formed, liquid crystal molecules 108a and 108b change their orientation directions on planes which are almost parallel to the surfaces of orientation films 107a and 107b. In this case, the birefringence occurs at liquid crystal molecules 108a and 108b. So the light directed from back light member 101 through polarizer 104a can transmit the polarizer 104b. As a result, a white screen (hereinafter represents a state in which the screen appears white) is achieved.
It is noted that although the liquid crystal having positive anisotropy of dielectric constant is used here, a liquid crystal having negative anisotropy of dielectric constant may be used. In this case, the orientation directions of liquid crystal molecules 108a and 108b in the case where the voltage is not applied to electrodes 106a and 106b are adjusted to have some angles with respect to electric field direction 110.
As shown in FIG. 18, when the light is directed in the direction which is perpendicular to the surfaces of polarizer 104a and 104b which are arranged with their transmission axes orthogonal to each other, the light in the perpendicular direction is shielded. However, when the light is directed in a direction which is diagonal with respect to the surfaces of polarizer 104a and 104b, that is, when an angle of view "THgr" is large, there would be some light transmitting through polarizer 104a and 104b (leakage light). Here, angle of view "THgr" is defined as shown in FIG. 22. FIG. 22 is a diagram showing a relation between angle of view "THgr" and electric field direction 110 generated in the liquid crystal display unit having the IPS mode.
Referring to FIG. 22, assume that an x axis is almost parallel to electric field direction 110. Then, a y axis is set on a plane which is almost parallel to the surfaces of polarizer 104a and 104b (with reference to FIG. 20). Further, a z axis is set in the direction which is perpendicular to an xy plane and also in the direction from polarizer 104a toward 104b. Then, an azimuth "PHgr" is defined on the xy plane with respect to the electric field direction. Finally, on a plane defined by the direction determined by azimuth "PHgr" and the z axis, an angle of view "THgr" with respect to the z axis is defined.
FIG. 23 is a graph showing a relation between a relative luminance and an angle of view of the light directed from the back light member. Referring to FIG. 23, even when angle of view "THgr" attains 80xc2x0, the relative luminance is about 50%, and the light is directed from the back light member diagonally with respect to the liquid crystal display.
Then, a relation between the angle of view, relative luminance and contrast ratio is measured in the liquid crystal display unit having the IPS mode shown in FIGS. 18 to 21. The result is shown in FIGS. 24 to 26.
FIG. 24 is a graph showing the relation between the relative luminance and the angle of view in the case of the white screen in the liquid crystal display unit having the IPS mode. FIG. 25 is a graph showing the relation between the relative luminance and the angle of view in the case of the black screen in the liquid crystal display unit having the IPS mode. FIG. 26 is a graph showing the relation between the contrast ratio and the angle of view in the liquid crystal display unit having the IPS mode.
Referring to FIG. 25, in the conventional liquid crystal display unit having the IPS mode shown in FIGS. 18 to 21, the relative luminance increases around points at which "THgr"=xc2x160xc2x0 in the case of the black screen. This is because the light which is diagonally directed with respect to the liquid crystal display 103 is detected as leakage light. Thus, although uniform luminance should be obtained at any angle of view in the case of the black screen, the luminance differs from the angle of view as the relative luminance increases around the points at which "THgr"=xc2x160xc2x0.
In addition, in this case, a transmission path in liquid crystal display 103 (with reference to FIG. 18) of the leakage light in the region with a large angle of view is longer than usual. As a result, retardation increases and the leakage light appears yellow. Therefore, in the region with a large angle of view, the color on the liquid crystal display unit is not sufficiently uniform.
As shown in FIG. 27, the above mentioned problem is particularly serious in the liquid crystal display unit such as the one having the IPS mode having the angle of view which is larger than that of the conventional liquid crystal display unit. FIG. 27 is a schematic diagram shown in conjunction with the problem of the region with the large angle of view. Referring to FIG. 27, while angle of view "THgr" of the conventional liquid crystal display unit is about xc2x140xc2x0, angle of view "THgr" of the above mentioned liquid crystal display unit having the IPS mode is about xc2x180xc2x0. The above mentioned problem is particularly serious in regions A and B having large angles of view shown in FIG. 27.
In addition, the light diagonally directed with respect to liquid crystal display 103 (with reference to FIG. 20) is scattered by a component of the liquid crystal display such as a spacer for increasing thickness of the region in which the liquid crystal is retained, and directed onto the front surface of liquid crystal display 103 from a region with disordered orientation of the liquid crystal near the spacer. As a result, as shown in FIG. 26, the contrast ratio at the front surface of liquid crystal display 103 (around the point at which "THgr"=0xc2x0) is as low as 95.
An object of the present invention is to enhance uniformity of luminance and color in a region with a large angle of view in a liquid crystal display unit having an angle of view which is larger than that of a conventional unit.
It is another object of the present invention to enhance a contrast ratio when viewed form the front side in a liquid crystal display unit having an angle of view which is larger than that of a conventional unit.
A liquid crystal display unit according to one aspect of the present invention is provided with a liquid crystal holding member, first and second electrodes and a light quantity adjusting member. The liquid crystal holding member has a display surface and a back surface. The first and second electrodes are formed on planes which are almost parallel to the back surface of the liquid crystal holding member, and form an electric field which is almost parallel to the back surface. The light quantity adjusting member is formed on the side of the back surface of the liquid crystal holding member and decreases a quantity of light diagonally directed from the back surface through the display surface.
Thus, a quantity of diagonally directed leakage light in the case of black screen decreases by decreasing the quantity of light diagonally directed through the display surface of the liquid crystal display unit. As a result, when viewed from a large angle of view, luminance in the case of the black screen is prevented from becoming larger than that in the case where the display surface is viewed from the front side, so that uniformity of luminance is enhanced. In addition, the quantity of light having a long transmission path in the liquid crystal holding member by diagonally directing the light through the display surface can be decreased. Therefore, the problem of the light which appears yellow when viewed from the large angle of view is minimized, so that uniformity of color is enhanced.
A liquid crystal display unit according to another aspect of the present invention is provided with a glass substrate, first and second electrodes, a liquid crystal holding member, a light quantity adjusting member. The first and second electrodes are formed on the glass substrate almost parallel to each other. The liquid crystal holding member is formed on the first and second electrodes and has a display surface and a back surface. The light quantity adjusting member is formed on the side of the back surface of the liquid crystal holding member and decreases a quantity of light diagonally directed from the back surface through the display surface.
Thus, a quantity of diagonally directed leakage light in the case of black screen decreases by decreasing the quantity of light diagonally directed through the display surface of the liquid crystal display unit. As a result, when viewed from a large angle of view, luminance in the case of the black screen is prevented from becoming larger than that in the case where the display surface is viewed from the front side, so that uniformity of luminance is enhanced. In addition, the quantity of light having a long transmission path in the liquid crystal holding member by diagonally directing the light through the display surface can be decreased. Therefore, the problem of the light which appears yellow when viewed from the large angle of view is minimized, so that uniformity of color is enhanced.
The liquid crystal display unit according to be above mentioned aspect and another aspect may further be provided with a polarizing member having a transmission axis. The polarizing member may be formed between the liquid crystal holding member and the light quantity adjusting member. The light quantity adjusting member may decrease the quantity of the light diagonally directed from the back surface through the display surface from an azimuth which differs from the transmission axis.
Thus, by decreasing the quantity of the diagonally directed light from the azimuth which differs from the transmission axis which readily allows generation of leakage light, the quantity of the leakage light in the case of the black screen can more effectively be reduced when viewed from a large angle of view. As a result, uniformity of luminance when viewed from the large angle of view is further enhanced.
In the liquid crystal display unit according to the above mentioned aspect and another aspect, the azimuth which differs from the transmission axis may form an angle of 45xc2x0 or 135xc2x0 with respect to the transmission axis.
Here, the light directed from the azimuth which forms the angle of 45xc2x0 or 135xc2x0 with respect to the transmission axis may turn to leakage light. As the quantity of the light directed from the azimuth which readily allows generation of leakage light can be decreased, uniformity of luminance and color when viewed from the large angle of view is particularly enhanced.
In the liquid crystal display unit according to the above mentioned aspect and another aspect, the light quantity adjusting member may include an orientation changing member for changing a direction of the diagonally directed light to a direction which is almost perpendicular to the display surface.
Thus, the quantity of the light which is directed almost perpendicularly to the display surface (in the direction which forms an angle of view of about 0xc2x0) can be increased. As a result, the contrast ratio when the display surface is viewed almost from the front side is enhanced.
In the liquid crystal display unit according to the above mentioned aspect and another aspect, the orientation changing member may be a condenser film.
The liquid crystal display unit according to the above mentioned aspect and another aspect may further be provided with a light conducting plate formed on the side of the back surface of the liquid crystal holding member, and the orientation changing member may be formed by processing the surface of the light conducting plate.
The liquid crystal display unit according to the above mentioned aspect and another aspect may further be provided with a light scattering member formed on the side of the display surface of the liquid crystal holding member.
As the direction of the part of the light which is directed perpendicularly to the display surface can be changed to the direction which is diagonal to the display surface by the light scattering member, luminance in the case of the white screen is enhanced when viewed from a large angle of view. As a result, when viewed from the large angle of view, while preventing leakage light in the case of the black screen, luminance in the case of the white screen is increased.
The liquid crystal display unit according to be above mentioned aspect may further be provided with a glass substrate having a surface almost parallel to the back surface. The glass substrate may be formed on a side of the back surface of the liquid crystal holding member. First and second electrodes may be formed on the surface of said glass substrate almost parallel to each other.
The liquid crystal display unit according to still another aspect of the present invention has an angle of view with respect to a display surface which is 40xc2x0 or larger, and is provided with a means having a display surface and a back surface for holding a liquid crystal, and a light quantity adjusting means formed on the side of the back surface for decreasing a quantity of light diagonally directed from the back surface through the display surface.
Thus, the quantity of leakage light in the direction which forms a large angle of view is decreased. In addition, even when the display surface is viewed from the direction which forms the large angle of view, increase in luminance by the leakage light in the case of the black screen can be prevented. As a result, uniformity of luminance is enhanced. Further, as a quantity of light having a long transmission path in the liquid crystal by diagonally directing the light through the display surface can decrease, even when the diagonally directed light appears yellow by retardation, the problem of the yellow light is minimized and uniformity of color is enhanced.
The liquid crystal display unit according to the above mentioned still another aspect may further be provided with a polarizing means having a transmission axis for allowing transmission of light of the light directed from the back surface through the display surface which has a vibration component almost parallel to the transmission axis. The light quantity adjusting means may decrease the quantity of the light which is diagonally directed from the back surface through the display surface from an azimuth which is different from the transmission axis.
Thus, the quantity of the light diagonally directed from the azimuth which is different from the transmission axis readily allowing generation of leakage light is decreased, so that the leakage light is more effectively be prevented when viewed from the large angle of view. As a result, uniformity of luminance and color is more effectively be enhanced when viewed from a region having a large angle of view.
The liquid crystal display unit according to the above mentioned still another aspect may further be provided with a means for scattering light directed from the back surface through the display surface.
Accordingly, as the direction of the part of the light which is directed perpendicularly to the display surface is changed to the direction which is diagonal to the display surface, luminance in the case of the white screen can be increased when viewed from the large angle of view. As a result, when viewed from the large angle of view, while preventing generation of leakage light in the case of the black screen, luminance in the case of the white screen is increased.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.